It is possible to obtain a much higher effective power gain from a laser amplifier by employing optical feedback with a cavity. The output of the amplifier can be controlled by injection of suitably tailored laser light into a cavity which resonates at the frequency of the injected light.
L. E. Erickson et al. report in Appl. Phys. Lett. 18, pages 433435 (1971) the spectral narrowing of dye laser output by injection of monochromatic radiation. The basic principles of laser injection locking are discussed by C. J. Buczek et al. in Proceedings of the IEEE, October 1973, pages 1411-1431. Injection locking is a technique employed for controlling the wavelength of a high power oscillator. For example, a narrow band injected signal induces the high power laser to emit a similarly narrowed frequency spectrum.
Magyar et al. reported in Appl. Phys. Letters 20, pages 406-408 (1972) the operation of a forced flashlamp pulsed dye laser oscillator, where the injection source was a flash lamp pulsed dye laser of 0.01 nm band width.
J. T. Turner et al. show in Applied Physics Letters 27, pages 441-443 (1975) that the line width of a pulsed dye laser can be drastically reduced without losing the tunability by injection locking the pulsed laser to an electro-optically tuned CW laser. The band width obtained was about 0.01 nm.
Danielmeyer in U.S. Pat. No. 3,676,799 issued July 11, 1972 discloses arrangements for stabilizing the frequency of single frequency laser by employing an etalon within the laser cavity.
W. Ganiel et al. in IEEE Journal of Quantum Electronics Vol. QE12, pages 704-716 (1976) analyze the mechanism of injection locking in pulsed dye laser systems and S. Blit et al. describe in Applied Physics 12, page 69 (1974) a tunable, single mode, injection locked flashlamp pumped dye laser.
The spectral narrowing of a flashlamp pulsed, waveguide dye laser by amplification of tunable narrow band CW radiation is described by P. Burlamacchi et al. in Optics Communications 17, pages 6-10 (1977). A flashlamp pumped dye laser can be controlled with a relatively low amount of CW narrow band radiation.
Pinard et al. in Optics Communications 20, pages 344-346 (1977) describe a dye laser arrangement in which super-position of both CW and pulsed excitation generates a single mode pulsed tunable laser light beam. In this work only a small part of the power of the pulsed frequency doubled Nd:YAG laser is used to excite two amplifying dye cells.
Hutchinson et al. in U.S. Pat. No. 4,019,157 discloses high power gas lasers which are seeded by a beam from a low power laser.
The conventional technique of injection locking has suffered from the difficulty that the injected signal must be injected into the resonator in such a way that it does not interfere with extraction of the laser emission from the amplifier. In addition, feedback from the amplifier mirrors into the master oscillator must be eliminated by some type of optical isolator. If this is not done, the oscillator will try to oscillate on a resonator mode which is out of resonance with the amplifier cavity. This tendency for off-resonance operation complicates the tuning of the two lasers and greatly reduces the efficiency of operation. For peak performance, both in output intensity and wavelength control, the oscillator and amplifier resonator must be adjusted for perfect resonance. This means that the resonator lengths must be controlled to approximately 1 part in 10.sup.7.
The previous injection-locking schemes for obtaining a narrow spectral output from a pulsed dye laser were accomplished by injecting monochromatic radiation from a separate (oscillator) laser into the cavity of the pulsed dye (amplifier) laser. The frequency of the high power output of the amplifier laser is locked to the frequency of the oscillator laser and has the same narrow spectral width characteristics as the oscillator laser. For such injection-locking schemes, however, problems concerning (a) the matching of the resonant frequencies of the oscillator and amplifier, (b) optional feedback between the oscillator and amplifier, and (c) the manner in which laser radiation is injected and extracted from the amplifier laser cavity must be solved for each setup.